If the court can make decisions based on Sharia law, couldn't they decide to base their decisions on Mosaic law just as well, and simply disregard civil law whenever it might suit them?
You're blinded by the "Sharia" aspects of the case. It's a more general issue, though. As it happens, judges are often asked to rule on matters involving the internal "laws" of various religions, with regard to matters regarding property and other matters.
Over the course of 150+ years of rulings on such matters, the courts have adopted a "deference" approach, whereby they'll defer to the internal rules of a religious body rather than make rulings that interfere with the First Amendment.
A classic expression of this principle is given in Watson vs. Jones (1871):
In such cases where the right of property in the civil court is dependent on the question of doctrine, discipline, ecclesiastical law, rule, or custom, or church government, and that has been decided by the highest tribunal within the organization to which it has been carried, the civil court will accept that decision as conclusive, and be governed by it in its application to the case before it.
It's clear that the Oklahoma law would violate that principle.